I do know, though, that the hymns listed for Mattins and Lauds in Hymn-melodies for the whole year from the Sarum service-books (which hymns are notated "as on the Feast of the Conception") are sung to the same tune; the hymn is called "O gloriosa femina" (or "O gloriosa domina") at Lauds, and "Quem terra, pontus, ethera" at Mattins. The tune is the same as, but the words do not match up to, the hymn I have for Lauds:

Here's a page from the Poissy Antiphonal that includes both of these hymns - but the melodies seem quite different:

1. O how glorious art thou, dazzling with light,stock of David, royal offspring!Thou dwellest in a sublime height, O Virgin Mary,Looking down on all the heavenly regions.

2. Thou, with the honor of being a virgin and mother,hast prepared for the Lord of Angels thy bosomas a sacred palace, thy most holy womb,from whence God took flesh, and was born Christ.

3. Thou, whom the whole world venerates and pays homage,before whom all now rightfully bend the knee,To whom we humbly beseech in our misery and darkness,coming before thee surrounded by the joy of pure light.

4. O Father of all lights, through this sacred Flamegive unto us thy only Begotten Son,who with Thee reigns brilliantly in the heavens,ruling and governing for all ages.Amen.

But now here's the real interest, at least for me: Letabundus, a Sequence Hymn, is sung at Second Vespers - a Christmas Sequence, that is! I've tried to find a recording of this, but as is usual with many of the Sequence hymns, it's just not out there [EDIT August 2009: Not true, anymore! See this post for an mp3 of Letabundus.]; most of the Sequences have fallen into disuse at this point. Here, though, is the score, with the English words (click the image for a larger version):